Fan Conducted Noise Reduction

ABSTRACT

Apparatus and methods for reducing unwanted conducted noise generated by a DC load  17.  Load  17  is powered by a voltage source  11.  A current sense  13  senses pulse current I fan  flowing through the load  17.  First converting means  14, 15, 16  converts the sensed pulse current I fan  to a correction voltage V p . The correction voltage V p  is then converted to a correction current I p , which offsets the deleterious effects of I fan . The current I s  flowing through the voltage source  11  equals I fan  plus the correction current I p .

TECHNICAL FIELD

The present invention pertains to reduction of conducted noise in direct current brushless fans used in network communication applications, and can also be used to reduce conducted noise emanating from other DC loads. The invention can be incorporated in the fan(s), or incorporated in the controlling electronics circuitry.

BACKGROUND ART

Direct Current (DC) fans are predominantly used to cool electronic equipment. The fans are typically brushless in design, to reduce cost and improve reliability. The commutation of the fan generates a current pulse on the power source. This current pulse varies in amplitude and frequency, proportional to the fan speed. Typically, the current pulse ranges from 0 Hz to 500 Hz, and may also exhibit harmonics well into the 10 kHz to 20 kHz range. Regulatory agencies are now requiring the conducted noise in the voice band to be below low levels, for example 9+10 logIc (dBrnC), where Ic is the maximum measured input current on the power port of the telecommunications equipment. Equipment that is designed to measure analog signals with C-Message weighting is specified to measure the noise. Typically, this is performed using a Transmission Impairment Measurement Set (TIMS) that is used for measurements on Plain Old Telephone Systems (POTS). The term dBrnC is a decibel relative measurement with C-Message weighting, and is specified as dBrnC=dBm+90. Thus, as an example, an input current of 2A results in a limit of 12 dBrnC. Applying the conversion formula results in an absolute value of 12 dBrnC−90=−78 dBm in the voice band range. Attenuating noise down to this low of a level can be achieved using a passive low-pass filter consisting of inductors, capacitors, and resistors on the power supply to the fan(s) or on the power supply port of the equipment cooled by the fan(s). The size of the filter components can increase significantly, depending on the current level required on the power port. Passive low-pass filters are widely used technology having moderate cost. However, using low pass filters causes significant pressure on the volume available to house all the components, depending on the level of attenuation and current rating.

Another method used to reduce the noise level is to use Pulse Width Modulation (PWM) on the fan power input. In the case where multiple fans are used in a system, the fans' PWM drives are driven out of phase with each other to minimize the additive current pulses. U.S. published patent application 2009/0278484 Al dated Nov. 12, 2009 shows one example of incorporating this method.

DISCLOSURE OF INVENTION

The present invention counteracts the effects of the undesired current pulses I_(fan) using an active filter to cancel out the current pulses I_(fan), so that a steady current I_(s) is seen at the fan power source 11. This method advantageously allows the use of small integrated circuits that can be incorporated in the housing(s) for the fan(s) 17, or else incorporated at the power source 11 driving the fan(s) 17. This can be achieved with small component volume and moderate cost.

The present invention measures the undesired current pulse I_(fan) and translates this pulse to voltage. Then a mathematical function V_(average)−V_(pulse)=V_(P) is performed using integrated circuits such as operational amplifiers. The correction voltage V_(p) is then presented to a voltage controlled current source or current sink 12, connected in parallel to the fan power source 11, that adds a counteracting current pulse I_(p) back to the source 11 to achieve a steady current I_(s). This operation is represented by the formula I_(s)=I_(fan)+I_(p). Thus, noise reduction at the system power input 11 is achieved.

The present invention performs the noise canceling independent of fan 17 speed. Thus, filtering down to low frequencies approaching 0 Hz can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other more detailed and specific objects and features of the present invention are more fully disclosed in the following specification, reference being had to the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a preferred embodiment of the present invention.

FIG. 2 illustrates an example of a circuit that can be used to perform the functions described in FIG. 1.

FIG. 3 illustrates a circuit simulation of V_(p)=V_(avg)−V_(pulse), using Linear Technologies LTspice IV circuit simulation software.

FIG. 4 illustrates a circuit simulation of I_(s)=I_(fan)+I_(p), using Linear Technologies LTspice IV circuit simulation software.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms, and should therefore not be construed as limited to the embodiments set forth herein. In the Figures, the dimensions of elements may be exaggerated for clarity of illustration. Like reference characters refer to like elements throughout.

The FIG. 1 block diagram describes the present invention configuration and control methodology. Normally, when no filtering is implemented, voltage source 11 is connected directly to the fan motor(s) 17. An unwanted current pulse, I_(fan) is generated due to the fan 17 commutation, and is seen by the voltage source 11.

In the present invention, a current sense 13 is placed in series between the voltage source 11 and fan motor(s) 17 to provide a current measurement of I_(fan). The current I_(fan) is then converted to voltage, to allow the above-mentioned mathematical function to be performed.

An average current to voltage converter 14, placed at the output of current sense 13, performs an average current to average voltage (V_(avg)) conversion. A pulse current to voltage converter 15 is also placed at the output of current sense 13, and performs a conversion of pulse current to pulse voltage (V_(pulse)). A V_(avg)−V_(pulse) subtraction module 16, located at the output of converters 14 and 15, then performs the mathematical function to generate the correction voltage V_(p).

V_(p) is then introduced to the control input of variable current source (or sink) 12, which converts the pulse correction voltage V_(p) to pulse correction current I_(p). The pulse correction current I_(p) is the antidote to the unwanted current pulse I_(fan), and is automatically added back to the source current I_(s) by the topology of the circuit by the relationship I_(s)=I_(fan)+I_(p). This results in a substantially steady current I_(p), and therefore minimizes noise generated by current ripple at voltage source 11.

The FIG. 2 schematic diagram represents one embodiment that can be used to implement the control methodology described in FIG. 1. There are many other possible and feasible methods to convert current to voltage to apply the mathematical function V_(p)=V_(avg)−V_(pulse) and there are many other possible and feasible methods to implement a voltage to current conversion to generate I_(p), other than the ones shown in FIG. 2.

Generally, the values for the various components shown on FIG. 2 are typical values.

Diode D1 is used to set the direction of current flow. It is typically a MBRS130L.

In FIG. 2, voltage source 11 is represented as V1, and the fan motor(s)' 17 pulse current I_(fan) is represented as I1. The current sense 13 is achieved by passing I_(fan) through resistor R4. This converts the current I_(fan) to a low voltage value. The value of R4 is typically kept very low to keep the voltage drop low, so that the operating voltage of the fan(s) 17 is not appreciably affected.

The low voltage value representing I_(fan) is then amplified using current sense amplifiers U2 and U3 (embodying modules 14 and 15, respectively). In this example, U2 and U3 are Linear Technology, Inc. LTC6101 High Voltage, High-Side Current Sense Amplifiers.

V_(avg) is generated by U2, and V_(pulse) is generated by U3.

The output voltage of U2 is determined by the equation V_(avg)=V_(R4)×R6/R5. V_(avg) is still a pulse at the output of U2. Thus, a capacitor C2 is used to smooth the pulse to represent the average voltage to the input of module 16.

The output voltage of U3 is determined by the equation V_(pulse)=V_(R4)×R3/R2.

The mathematical function V_(p)=V_(avg)−V_(p) (module 16) is implemented using a typical differential amplifier, U5, with accompanying components R7, R8, R9, R10, C6, and C7. U5 is typically a Linear Technology 1784. The differential amplifier U5 can be embodied by many other known implementations.

V_(p) is then converted to current I_(p) (at module 12) using a current feedback amplifier, U4, with accompanying components R11, R13, and C5. These components implement variable current source 12. U4 is typically a Linear Technology 1206. The current source 12 (which can also be a current sink) can also be embodied by many other known implementations.

FIG. 3 and FIG. 4 are simulations of the circuit shown in FIG. 2, using Linear Technology, Inc. LTSpice IV circuit simulation software.

FIG. 3 shows V_(p)=V_(avg)−V_(pulse) waveforms.

FIG. 4 shows I_(s)=I_(fan)+I_(p) waveforms, and illustrates that the current ripple on I_(s) is greatly reduced by the present invention, thus reducing noise levels measured on the voltage source 17.

The above description is included to illustrate the operation of preferred embodiments, and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the present invention. For example, the invention can be used to reduce unwanted conducted noise generated by any DC load, not just fans. 

What is claimed is:
 1. Apparatus for reducing unwanted conducted noise generated by a DC load, said apparatus comprising: a current sense adapted to sense pulse current flowing through the load; coupled to the current sense, first converting means for converting the sensed pulse current to a correction voltage; and coupled to the first converting means, second converting means for converting the correction voltage to a correction current.
 2. The apparatus of claim 1 wherein the load comprises at least one fan having commutation.
 3. The apparatus of claim 1 wherein the current sense comprises a resistor.
 4. The apparatus of claim 1 wherein the first converting means comprises an average current to voltage converter having an output V_(avg) and a pulse current to voltage converter having an output V_(pulse).
 5. The apparatus of claim 4 wherein the average current to voltage converter and the pulse current to voltage converter each comprise a current sense amplifier.
 6. The apparatus of claim 4 wherein the first converting means further comprises means for subtracting V_(pulse) from V_(avg) to produce the correction voltage.
 7. The apparatus of claim 6 wherein the subtracting means comprises a differential amplifier.
 8. The apparatus of claim 1 wherein the second converting means comprises a current feedback amplifier.
 9. The apparatus of claim 1 further comprising a voltage source coupled to, and providing power to, the DC load, wherein the current flowing through the voltage source equals the sum of pulse current flowing through the load plus the correction current.
 10. The apparatus of claim 9 wherein the second converting means is connected in parallel to the voltage source.
 11. A method for sensing and reducing DC load ripple current in order to reduce conducted noise, said method comprising the steps of: sensing the DC load ripple current; converting the sensed ripple current to a correction voltage; applying a mathematical function to convert the correction voltage into a correction current; and applying the correction current to offset the DC load ripple current.
 12. The method of claim 11 wherein the DC load comprises at least one fan having commutation.
 13. The method of claim 11 wherein the step of sensing DC load ripple current comprises passing the DC load ripple current through a resistor.
 14. The method of claim 11 wherein the step of converting the sensed ripple current to a correction voltage comprises using an average current to voltage converter to produce a voltage V_(avg), and using a pulse current to voltage converter to produce a voltage V_(pulse).
 15. The method of claim 14 wherein the average current to voltage converter and the pulse current to voltage converter each comprise a current sense amplifier.
 16. The method of claim 14 wherein the step of converting the sensed ripple current to a correction voltage further comprises the substep of subtracting V_(pulse) from V_(avg) to produce the correction voltage.
 17. The method of claim 16 wherein the subtracting substep is performed using a differential amplifier.
 18. The method of claim 11 wherein the applying step comprises applying the correction voltage to an input of a current feedback amplifier.
 19. The method of claim 18 wherein the current feedback amplifier is connected in parallel to a voltage source.
 20. The method of claim 11 wherein the correction current is added to the DC load ripple current for presentation to a voltage source coupled to and powering the DC load, and the correction current serves to offset unwanted noise effects of the DC load ripple current. 